Arrangement for measuring a time variable magnetic field using the faraday effect

ABSTRACT

An arrangement for measuring a time-variable magnetic field in order to measure, for example, the current flowing in a highvoltage line, utilizes a measuring head subjected to a magnetic field proportional to the line current which encloses a magnetooptical element in the form of a Faraday cell that functions in conjunction with an optical divider in the form of a Koesters prism to form a two-beam interferometer. A laser produced monochromatic light beam fed to the measuring head over an optical fiber bundle passes through a polarizing filter onto the divider plane of the Koesters prism where it is split into two partial beams. One beam passes through a first quarter-wave plate in which it is circularly polarized in a dextrorotary sense, thence through the Faraday cell where it undergoes multiple reflection in the magnetic field and thence through a second quarter-wave plate back to the divider plane of the Koesters prism. The other beam passes through the same path but in the opposite direction, passing first through the second quarter-wave plate where it is polarized in a levorotary sense, and ultimately arriving back at the divider plane. The two returning beams combine in the divider plane of the Koesters prism to form an outgoing beam consisting of two 90* phase-displaced components modulated in intensity as a function of the variation in the magnetic field. The two phase-shifted beam components are passed respectively through optical fiber bundles to a processing circuit where they are converted into correspondingly modulated electrical signal components which are then demodulated to a final output electrical signal constituting a measure of the current in the high-voltage line.

(United States Patent [72] Inventor Friedrick K. Von Willisen Zurich,Switzerland [2]] Appl. No. 818,290 [22] Filed Apr. 22, 1969 [45]Patented Nov. 16, 1971 [73] Assignee Aktiengesellschaft Brown, Boveri &Cie

Baden, Switzerland [32] Priority Apr. 23, 1968 [3 3] Switzerland [31]60005/68 [54] ARRANGEMENT FOR MEASURING A TIME VARIABLE MAGNETIC FIELDUSING THE FARADAY EFFECT 6 Claims, 2 Drawing Figs.

[5 2] U.S. Cl 324/96, 324/43 L, 350/151 [51] lnt.Cl G0lr 31/00 [50]Field of Search 324/43 L, 96; 350/151, 158

[5 6] References Cited UNITED STATES PATENTS 3,495,892 2/1970 Dailey350/151 3,502,978 3/1970 Bernard 324/96 Primary Examiner-Michael .l.Lynch Artorney- Pierce, Scheffler & Parker ABSTRACT: An arrangement formeasuring a time-variable magnetic field in order to measure, forexample, the current flowing in a high-voltage line, utilizes ameasuring head subjected to a magnetic field proportional to the linecurrent which encloses a magneto-optical element in the form of aFaraday cell that functions in conjunction with an optical divider inthe form of a Koesters prism to form a two-beam interferometer. A laserproduced monochromatic light beam fed to the measuring head over anoptical fiber bundle passes through a polarizing filter onto the dividerplane of the Koesters prism where it is split into two partial beams.One beam passes through a first quarter-wave plate in which it iscircularly polarized in a dextrorotary sense, thence through the Faradaycell where it undergoes multiple reflection in the magnetic field andthence through a second quarter-wave plate back to the divider plane ofthe Koesters prism. The other beam passes through the same path but inthe opposite direction, passing first through the second quarter-waveplate where it is polarized in a levorotary sense, and ultimatelyarriving back at the divider plane. The two returning beams combine inthe divider plane of the Koesters prism to form an outgoing beamconsisting of two 90 phase-displaced components modulated in intensityas a function of the variation in the magnetic field. The twophase-shifted beam components are passed respectively through opticalfiber bundles to a processing circuit where they are converted intocorrespondingly modulated electrical signal components which are thendemodulated to a final output electrical signal constituting a measureof the current in the high-voltage line.

/ LASER PRODUCED DUAL FIBER BUNDLES TO PHOTU DETECTORS 7 1/7 monocuaommcLIGHT BEAM 73 I9 ARRANGEMENT FOR MEASURING A TIME VARIABLE MAGNETICFIELD USING THE FARADAY EFFECT The present invention concerns anarrangement for measuring a time-variable magnetic field using theFaraday effect.

Such arrangements are used, for example, for measuring the electriccurrent in high voltage lines, where the current to be measured producesa magnetic field in a magneto-optical element at high voltage potentialand where the rotation of the polarization plane of a beam passingthrough this magneto-optical element is determined. In a known method ofthis type, the rotation of the polarization plane of the beam isconverted into a variation of the optical radiation intensity. Therelation between angle of rotation and light intensity is not linear,however, and is ambiguous for angles of rotation of more than 11/2. Alimitation to angles of rotation of less than 1r/2 would have theresult, however, that extremely small angles of rotation would have tobe resolved in the interest of a sufficiently wide dynamic range. Inanother known method, the problem of the nonlinearity is avoided bycompensating the polarization rotation of the beam produced by the fieldto be measured in another magnetic-optical element, the compensatingmagnetic field being a measure for the field to be measured. Theapplication of this method to the measurement of electric currentnecessitates, however, relatively expensive devices in those cases whererapid variations of the current intensity, such as in a short circuit,for example, must be reproduced correctly, because of the high powerconsumption of the regulating device.

The object of the invention is an arrangement for measuring thevariation of a magnetic field, using the Faraday effect, where themeasuring accuracy and time resolving power achieved with relativelysimple and inexpensive means are substantially higher than in the knownmethods, while the device is to a great extent insensitive to mechanicalvibrations andtemperature influences.

The arrangement according to the invention is characterized by the factthat a monochromatic beam is split in a divider arrangement in itsintensity into two partial beams, that the outgoing split beams are eachpolarized circularly by an associated circular polarizer in oppositesense, and that the split beams follow the same light path, startingfrom the divider arrangement and leading back to it, in oppositedirection, that two circularly polarized partial beams pass through amagneto-optical element in this light path in which the difference ofthe optical path lengths of the two split beams is varied proportionallyto the field-strength of the magnetic field prevailing there, and thatthe returning split beams interfere with each other in the dividerarrangement, producing an outgoing an outgoing beam, modulated in itsintensity, whose intensity variation is measured and from which anoutput signal is derived that is proportional to the difference of theoptical path lengths of the split beams.

The invention will be described below more fully on the basis of anarrangement for measuring the variations in time and amplitude of acurrent flowing in a high-voltage transmission line in conjunction withthe accompanying drawings wherein:

FIG. 1 is a longitudinal section of the measuring head including itsconnections to the incoming and outgoing optical fiber bundles and FIG.2 is a block diagram of the electrical circuit at the receiving end ofthe optical fiber bundles, said electrical circuit functioning toprocess the electro-optical output of the measuring head.

With reference now to FIG. 1 there is depicted an interferometricmeasuring head in longitudinal section. This measuring head contains ina gastight housing 1 a magneto-optical element 2 which in conjunctionwith a Koesters prism 7 forms a two-beam interferometer. The cylindricalpart 3 of the housing 1, inside of which the magneto-optical element 2is arranged, is enclosed by a magnetic coil 4, e.g. constituted by a fewturns of the high voltage conductor carrying the electrical current ofthe line which produces the time variable magnetic field to which theFaraday cell is subjected. The magneto-optical element 2 consists of aprism of magneto-optical material which is reflection-coated on itsplane-parallel end faces 5, 6. The reflective coating has on the endface 6 facing the measuring interferometer at the edge two recesses forthe passage of the beams.

The measuring interferometer contains a Koesters prism 7 acting as adivider arrangement, which is so arranged that its divider plane 8 issubstantially perpendicular to the end faces 5, 6 of the magneto-opticalelement. On the hypotenuse of the Koesters prism 7 perpendicular to thedivider plane 8 are arranged two A/4 plates 9, l0 acting as circularpolarizers in conjunction with polarization filter 15. An entrance lens11 and an exit lens 12 respectively are arranged in apertures of thehousing bottom. Between the entrance lens 11 and the outlet lens 12respectively on the one hand, and the Koesters prism 7, on the otherhand, are arranged a polarization filter each, 15 and 16 respectively,and a reflecting mirror, 17 and 18 respectively.

A single or double optical fiber 21, 22 leads to the apertures 13, 14 ofthe housing bottom over sockets 19, 20. For separating two beams passingthrough the optical fiber 22, a diaphragm 37 with two apertures isprovided inside the socket 20 in the focusing plane of the exit lens 12.

The measuring head, containing coil 4 which carries the current or aportion thereof in the high-voltage line, is connected, on the one hand,over the optical fiber 21 with a gas laser which produces amonochromatic light beam and, on the other hand, over the two lightconductor paths of the double optical fiber 22 with two photo detectors.The output signals of these photo detectors are fed to a processingcircuit to be described below. The gas laser and the photo detectorswith the processing circuit have ground potential. I

The method of operation of the arrangement described above is asfollows:

The monochromatic beam produced in the gas laser is fed over the opticalfiber 21, the entrance lens 11, the polarization filter 15 and thereflecting mirror 17 to the Koesters prism 7, on the divider plane 8 ofwhich it is split into two partial beams. One split beam passes throughA4 plate 9 in which it is circularly polarized in a dextrorotary sensein the closed optical path 23, the magneto-optical element 2, on thereflective coated end faces 5, 6 of which it undergoes a multiplereflection, and arrives finally by way of M4 plate 10 back at thedivider plane 8 of the Koesters prism 7. The other split beam passesthrough the same closed path 23 in the opposite direction, but ispolarized circularly in the M4 plate 10 in a levorotary sense. The tworeturning split beams combine in the divider plane 8 of the Loestersprism 7 to an outgoing beam, which is fed over the reflecting mirror 18and the polarization filter 16 to the exit lens 12. As long as there isno magnetic field, the effective optical paths of the two split beamspassing through the light path 23 in opposite directions are equal, sothat the outgoing beam does not show any interference phenomena varyingin time. When a magnetic field appears, the effective light path becomesdifferent in the magneto-optical element 2 for the first split beampolarized in a dextrorotatory sense from that of the second split beampolarized in a levorotary sense. The time-dependent difference (it/2w) I(t) of these effective light paths is strictly proportional, as it canbe shown, to the field strength H(t) of the time variable magnetic fieldto be measured, where A is the optical wavelength in the vacuum. In atime-variable magnetic field, the outgoing beam shows an interferometricmodulation corresponding to this optical path difference.

For the determination of the phase function I (1) from the modulatedoutgoing beam it is therefore necessary to determine the phase of themodulation in addition to the intensity value. To this end, two beams,phase-shifted by in their modulation, are produced in the measuring head(FIG. 1). This is done by adjusting the magneto-optical element 2 in thedrawing plane by a very small angle with respect to the Koesters prism 7which causes the two interfering beams to be offset relative to oneanother whereby parallel interference fringes are produced withsinusoidally changing intensity. One thus obtains a relative shear ofthe wave fronts of the two split beams passing through the light path 23in opposite direction, Due to this shear, a sinusoidal intensityvariation is produced in the beam issuing from the divider plane 8 ofthe Koesters prism 7, which extends over the beam cross section in onedirection. From this beam are coupled out, via two slit-shaped aperturesof the diaphragm 37, two exit beams which are phase-shifted in theirmodulation by 90 and which are fed each over an optical path of thedouble optical fiber 22 to an associated photo detector 24, 25.

The processing circuit and its method of operation will be describedwith reference to FIG. 2.

The detector currents i i of the photodetectors 24, 25, after the DCcomponents have been eliminated and after amplification in the stages26, 27 have, the standardized form:

In an intermediate frequency oscillator 28 with connected phase-shifter29 are produced two oscillator signals g,, g, likewise phase-shifted by90, which have the frequency w, and are of the standardized form g,= cosm t g sin (o The oscillator frequency w, must satisfy the conditions muand These oscillator signals are multiplied in the ring or balancedmodulator stages 30, 31 with the signals i, and i respectively, and theproducts fed to an additive networks 33 whose output signal G==i,',fl gcos [(11: 1:00)] has the form of a true phasemodulated carrier frequencysignal with the frequency deviation AF=(1/2rr)(d/dt) l (t) From thiscarrier frequency signal 0 is derived, by frequency demodulation in theFM-discriminator 35 and subsequent integration in the integration stage36 a signal of the form DU), i.e., the argument of sin (t) or cos 1 (t),respectively, which is a measure of the current intensity to be measuredin the high-voltage line.

lclaim:

1. In an arrangement for measuring a time-varied magnetic field, thecombination comprising an optical beam divider, means directing amonochromatic light beam into said divider to produce first and secondoutgoing beam components, first and second circular polarizers initiallytraversed by said first and second beam components respectively, saidpolarizers being oriented such that the beam components issuinginitially therefrom are circularly polarized in opposite directionsrespectively, a magneto-optical element exhibiting a Faraday effect andwhich is located in and subjected to the magnetic field to be measured,said magneto-optical element being transversed by said circularlypolarized beam components along the same path but in opposite directionswhereby due to the Faraday effect a difference in the optical pathlength between said beam components proportional to said time variablemagnetic field is produced, said first and second beam components beingthen passed through said second and first circular polarizersrespectively and brought to interference in said divider thereby toproduce an intensity-modulated light beam, and means includingphotodetector means deriving from said intensity modulated light beamsinusoidal electric signals the arguments of which are proportional tothe magnetic field to be measured.

2. In an arrangement for measuring the variation in a magnetic field inorder to measure, for example, the current flowing in a high voltageline, the combination comprising a measuring head subjected to amagnetic field proportional to the line current, said measuring headenclosing a magneto-optical element constituted by a Faraday cell whichin cooperation with an optical beam divider constituted by a Koestersprism forms a two-beam interferometer, means directing a monochromaticlight beam into said prism to produce at the divider plane therein firstand second outgoing beam components, first and second circularpolarizers initially traversed by said first and second beamsrespectively, said polarizers being oriented such that the beamcomponents issuing initially therefrom are circularly polarized inopposite directions respectively, said polarized beam components beingdirected into said Faraday cell for multiple reflection between theplane parallel end surfaces thereof along the same path but in oppositedirections whereby due to the Faraday effect a difference in the opticalpath length between said beam com ponents proportional to the variationin said magnetic field is produced, said first and second beamcomponents after issuing from said Faraday cell being passed throughsaid second and first circular polarizers respectively and brought tointerference in said prism to produce an intensity-modulated light beamfrom which sinusoidal electric signals are derived via means includingphotodetector means, the arguments of the sinusoidal signals beingproportional to the magnitude of the field to be measured,

3. An arrangement as defined in claim 2 for measuring a variablemagnetic field wherein said circular polarizers are constituted byquarter-wave plates secured to one face of said Koesters prism ingeneral parallel relation to the end faces of said Faraday cell, andwherein the incident light beam is plane polarized in a directionparallel to the divider plane of said Koesters prism.

4. An arrangement as defined in claim 2 for measuring a variablemagnetic field produced by current flow on a highvoltage line whereinsaid measuring head is subjected to the potential of the line andwherein said means including said photodetector means for derivingsinusoidal electric signals from said intensity-modulated light beamhave ground potential and are connected to said measuring head by way oflighttransmitting fiber bundles.

5. An arrangement as defined in claim 2 for measuring a variablemagnetic field wherein said circular polarizers are constituted byquarter-wave plates secured to one face of said Koesters prism ingeneral parallel relation to the end faces of said Faraday cell, andwherein the incident light beam is plane polarized in a directionperpendicular to the divider plane of said Koesters prism.

6. An arrangement as defined in claim 2 for measuring a variablemagnetic field wherein said circular polarizers are constituted byquarter-wave plates secured to one end face of said prism and whereinsaid Faraday cell is rotated by a small angle relative photodetectors,said prism whereby the wave fronts of said first and second beamcomponents interfering in said prism are sheared thus to produce asinusoidal intensity distribution over the cross section of saidintensity-modulated beam issuing therefrom, and which further includestwo slit diaphragms disposed in the path of said intensity-modulatedlight beam issuing from said prism such that two light beamsphase-shifted by in modulation are separated, a pair of photodetectors,a pair of fiber bundles directing said phaseshifted light beamsrespectively to said photodetectors, a pair of amplifiers connectedrespectively to the outputs of said photodetectors, a pair of modulatorstages correlated respectively to said amplifiers, an intermediatefrequency oscillator, a phase shifter producing a phase shift of 90, thetwo inputs to one of said modulator stages being constitutedrespectively by the output of the correlated amplifier and the outputdirectly from said intermediate frequency oscillator, and the two inputs to the other modulator stage being constituted respectively by theoutput of the correlated amplifier and an output from said intermediatefrequency oscillator following passage through said phase shifter, anadditive network to which the outputs from said two modulator stages areconnected, said additive network producing a carrier signalphase-modulated proportional to the magnetic field to be measured, and afrequency discriminator and integrator following said additive networkfor demodulating said phase-modulated carrier signal to determine thephase which is proportional to the magnetic field to be measured.

, EDWARD PLFLETCIEEIR ,JR. Aci esting Officer Patent No. 3: s39

Dated Nov. 16, 1971 Inver.tor(sb Friedrich: K Von Williaen r. iscertified that error appears in the. above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Claim 6 line 5 "photodetectoi's" should be cancelled, and the ward toahould :qe inserted.

ignecl' and sealed this 23rd day of May 1972.

ROBERT. GOTTSCHALK Commissioner Of Patents

1. In an arrangement for measuring a time-varied magnetic field, thecombination comprising an optical beam divider, means directing amonochromatic light beam into said divider to produce first and secondoutgoing beam components, first and second circular polarizers initiallytraversed by said first and second beam components respectively, saidpolarizers being oriented such that the beam components issuinginitially therefrom are circularly polarized in opposite directionsrespectively, a magneto-optical element exhibiting a Faraday effect andwhich is located in and subjected to the magnetic field to be measured,said magneto-optical element being transversed by said circularlypolarized beam components along the same path but in opposite directionswhereby due to the Faraday effect a difference in the optical pathlength between said beam components proportional to said time variablemagnetic field is produced, said first and second beam components beingthen passed through said second and first circular polarizersrespectively and brought to interference in said divider thereby toproduce an intensitymodulated light beam, and means includingphotodetector means deriving from said intensity modulated light beamsinusoidal electric signals the arguments of which are proportional tothe magnetic field to be measured.
 2. In an arrangement for measuringthe variation in a magnetic field in order to measure, for example, thecurrent flowing in a high voltage line, the combination comprising ameasuring head subjected to a magnetic field proportional to the linecurrent, said measuring head enclosing a magneto-optical elementconstituted by a Faraday cell which in cooperation with an optical beamdivider constituted by a Koesters prism forms a two-beam interferometer,means directing a monochromatic light beam into said prism to produce atthe divider plane therein first and second outgoing beam components,first and second circular polarizers initially traversed by said firstand second beams respectively, said polarizers being oriented such thatthe beam components issuing initially therefrom are circularly polarizedin opposite directions respectively, said polarized beam componentsbeing directed into said Faraday cell for multiple reflection betweenthe plane parallel end surfaces thereof along the same path but inopposite directions whereby due to the Faraday effect a difference inthe optical path length between said beam components proportional to thevariation in said magnetic field is produced, said first and second beamcomponents after issuing from said Faraday cell being passed throughsaid second and first circular polarizers respectively and brought tointerference in said prism to produce an intensity-modulated light beamfrom which sinusoidal electRic signals are derived via means includingphotodetector means, the arguments of the sinusoidal signals beingproportional to the magnitude of the field to be measured.
 3. Anarrangement as defined in claim 2 for measuring a variable magneticfield wherein said circular polarizers are constituted by quarter-waveplates secured to one face of said Koesters prism in general parallelrelation to the end faces of said Faraday cell, and wherein the incidentlight beam is plane polarized in a direction parallel to the dividerplane of said Koesters prism.
 4. An arrangement as defined in claim 2for measuring a variable magnetic field produced by current flow on ahigh-voltage line wherein said measuring head is subjected to thepotential of the line and wherein said means including saidphotodetector means for deriving sinusoidal electric signals from saidintensity-modulated light beam have ground potential and are connectedto said measuring head by way of light-transmitting fiber bundles.
 5. Anarrangement as defined in claim 2 for measuring a variable magneticfield wherein said circular polarizers are constituted by quarter-waveplates secured to one face of said Koesters prism in general parallelrelation to the end faces of said Faraday cell, and wherein the incidentlight beam is plane polarized in a direction perpendicular to thedivider plane of said Koesters prism.
 6. An arrangement as defined inclaim 2 for measuring a variable magnetic field wherein said circularpolarizers are constituted by quarter-wave plates secured to one endface of said prism and wherein said Faraday cell is rotated by a smallangle relative to said prism whereby the wave fronts of said first andsecond beam components interfering in said prism are sheared thus toproduce a sinusoidal intensity distribution over the cross section ofsaid intensity-modulated beam issuing therefrom, and which furtherincludes two slit diaphragms disposed in the path of saidintensity-modulated light beam issuing from said prism such that twolight beams phase-shifted by 90* in modulation are separated, a pair ofphotodetectors, a pair of fiber bundles directing said phase-shiftedlight beams respectively to said photodetectors, a pair of amplifiersconnected respectively to the outputs of said photodetectors, a pair ofmodulator stages correlated respectively to said amplifiers, anintermediate frequency oscillator, a phase shifter producing a phaseshift of 90*, the two inputs to one of said modulator stages beingconstituted respectively by the output of the correlated amplifier andthe output directly from said intermediate frequency oscillator, and thetwo inputs to the other modulator stage being constituted respectivelyby the output of the correlated amplifier and an output from saidintermediate frequency oscillator following passage through said phaseshifter, an additive network to which the outputs from said twomodulator stages are connected, said additive network producing acarrier signal phase-modulated proportional to the magnetic field to bemeasured, and a frequency discriminator and integrator following saidadditive network for demodulating said phase-modulated carrier signal todetermine the phase which is proportional to the magnetic field to bemeasured.